1. Field of the Invention
The invention relates to a beam defining system for an electron accelerator. The beam defining system has an adjustable collimator and an accessory holder for the attachment of, among other items, an electron applicator formed of an applicator wall enclosing an electron-beam cone and a frame-shaped spacer which is attached at the applicator wall and which can be brought into contact with the patient.
2. Description of the Prior Art
When patients are subjected to electron beams due to the scattering of electrons in air, it is necessary to limit the electron-beam cone by an enclosed path bridging the gap between the beam defining system and the patient. It is known in the art to attach electron applicators to an accessory holder of the beam defining system. These electron applicators are placed into direct contact with the patient. They have the further function of fixing the distance of the patient from the beam defining system, a distance which must be precisely maintained for dosage computations. A few electron applicators of this type were previously made transparent to simplify positioning. One of the disadvantages of prior art devices was the lack of direct accessibility to the exposed field such as the skin surface of the patient with markings applied thereto. This disadvantage has been eliminated in such a way that electron applicators have been shortened 10 to 20 cm and a frame-shaped spacer is mounted to the electron applicator by supports in order to set the desired spacing. In the case of such an electron applicator, however, there is the disadvantage that the beam quality in the marginal regions of the beam cone is effected by increased low energy portions of the beam. Due to the energy-dependent range of the electrons, a high dosage decrease in the marginal regions of the beam cone is present deep within the patient's tissue as compared to the center regions of the beam cone. It is also considered a disadvantage that even minor deviations of the symmetry axis of the applicator from the center beam, i.e., the axis of the beam cone, causes clearly changed dosages in the marginal regions of the beam cone.